In optical communications systems and, more particularly, in long-haul communications systems, sources capable of emitting single-frequency signals are of primary interest in order to reduce as much as possible signal distortion due to the different propagation rates of the different frequencies. This condition is essential in the case of high-speed direct source modulation and for use in coherent communications systems.
Semiconductor lasers in which optical feedback is obtained by other mechanisms than multiple reflections between the mirrors delimiting the laser cavity are examples of sources meeting the above requirement. In those lasers a selection of the oscillation free (or modes) is obtained without resorting to external components, so that the laser can be fabricated by integrated-optics circuit technology. Moreover, owing to the absence of the end mirrors, the devices are suitable for integration with other components of an optical communications system. Some examples of such lasers are Distributed Bragg Reflector (DBR) lasers and Distributed-Feedback (DFB) lasers. The latter lasers are of simpler manufacture and hence are presently preferred.
Generally, in DFB lasers, optical feedback is obtained thanks to a periodic spatial variation in the effective refractive index (i.e. the refractive index presented by the whole structure for the guided radiation) in the light propagation direction (longitudinal direction). This variation is caused by a grating extending across the whole cavity on or beneath the laser active layer. This optical feedback mechanism is commonly known as "index coupling" and exploits the fact that each refractive index change is accompanied by a weak reflection in the guided radiation. By a suitable choice of the grating period, back reflection will be obtained, and the grating acts as a wavelength-selective mirror, which reflects only the wavelengths closer to that which meets the Bragg condition ##EQU1## where .LAMBDA.=grating period
.lambda.B Bragg wavelength PA1 m=grating order PA1 n.sub.v =mode refractive index
In practice, .lambda..sub.B is chosen so as to be coincident with the emission wavelength of the laser active layer.
Yet, index-coupled DFB lasers are not per se monomode sources, and their behavior, in terms of emitted modes, is highly sensitive to reflections on the end facets. More particularly, if those facets are left untreated, the oscillation modes of the laser depend on the relative positions of the facets with respect to the grating spatial phase, which position is entirely casual, since it is impossible to exactly determine at which point a grating will be cut upon manufacturing the individual devices. If the laser facets are covered with antireflecting coatings, the laser will steadily oscillate on two modes symmetrical with respect to Bragg wavelength. In the latter case monomodality can be achieved by causing the rays propagating in the laser to undergo a quarter-wave phase shift in the central grating zone. This phase shift is obtained by eliminating a groove of the grating in such a zone, which operation is rather complicated from the technological standpoint. The operations necessary to manufacture a grating presenting the phase shift and to apply the antireflecting coatings cause such a cost increase that generally, in the industrial production of such lasers, it is preferred to keep which are monomode by fabrication and to discard the others. Notwithstanding the elimination of a considerable proportion of the production, this approach is still advantageous from the economic standpoint.
The problems of high costs or wastes can be solved by manufacturing a DFB laser where a longitudinal periodic gain variation (gain coupling) occurs instead of a periodic refractive-index variation. It has been theoretically shown (H. Kolgenick, C. V. Shank: "Coupled Wave Theory of Distributed Feedback Lasers", Journal of Applied Physics, Vol. 43, No. 5, May 1972) that a DFB laser is intrinsically a monomode structure and is relatively insensitive to facet reflections, so that no expensive machining such is required for phase-shift introduction in the grating and application of antireflecting coatings is necessary.
An example of gain-coupled DFB laser is described by Y. Luo et al. in the paper entitled "Gain Coupled DFB Semiconductor Laser Having Corrugated Active Layer" presented at the International Conference on Solid Slate Devices and Materials, Tokyo, August 1988, paper 20DPB-2, and issued at pages 328-330 of the proceedings of such a Conference. That paper describes a GaAs/GaAlAs laser, comprising, between conventional confinement layers of GaAlAs (lower and upper claddings), a corrugated layer or grating (pattern-providing layer), this too made of GaAlAs, but with such relative proportions of the constituent elements as to give a refractive index which is high if compared to the lower and upper claddings and is close to that of the active layer. The grating is followed by a further corrugated layer, always made of GaAlAs ("buffer layer"), who index is low but slightly higher than that of the lower and upper claddings. The active layer of undoped GaAs is deposited on the buffer layer in such as to obtain a planar structure. In this way a periodic thickness variation is obtained which causes a periodic gain variation. The refractive indices of the various layers and the heights of the teeth of the pattern-providing layer and the active layer are chosen so that the effective refractive index remains constant in longitudinal direction, so as to obtain a laser with pure gain-coupling.
This known structure presents a number of disadvantages due to the presence of a massive active layer, which does not allow attainment of high values not only of the absolute gain, but also of the differential gain (dg/dN, where g=absolute gain, N=number of the injected carriers). As is known, the higher the differential gain, the better the spectral line-width characteristics of the laser and more generally the dynamic properties of the device (frequency behavior, frequency modulation, etc.).